GOLD NANOPARTICLES: SYNTHESIS,PROPERTIES AND APPLICATIONS

 

GOLD NANOPARTICLES

In recent years, nanotechnology has been the topic of extensive research and a great deal of interest among researchers. The manufacturing and utilization of nanoparticles (NPs) has expanded dramatically as a result of the rapid development of nanotechnology. The term “nano” is derived from the Greek word “nanos” which means small and it is used as the prefix for one billionth part (10^-9). According to American Society for Testing and Materials (ASTM international 2006), nanoparticles are those particles which have two or more than two dimensions and are in the size range of 1 - 100 nm. These particles have special and enhanced physical and chemical properties as compared to their bulk materials due to their large reactive and exposed surface area and quantum size effect as a result of specific electronic structures. It can be broadly classified into semiconductor quantum dots,magnetic nanoparticles,polymeric nanoparticles,carbon bases nano structures and metallic nanoparticles. These particles have been widely used in many fields such as electronics, photochemical, biomedicine and chemistry.

 Gold nanoparticles (AuNPs) are one of the most important nanoparticles, and they have been widely used for medical and non-medical applications as ideal material because of their unique distinct features: inert, biocompatible, and especially due to low toxicity. Properties of gold nanoparticles are different from its bulk form because bulk gold is yellow solid and it is inert in nature while gold nanoparticles are wine red solution and are reported to be anti-oxidant. Inter particle interactions and assembly of gold nanoparticles networks play key role in the determination of properties of these nanoparticles.Gold nanoparticles exhibit various sizes ranging from 1 nm to 8 µm and they also exhibit different shapes such as spherical, sub-octahedral, octahedral, decahedral, icosahedral multiple twined, multiple twined, irregular shape, tetrahedral, nanotriangles, nanoprisms, hexagonal platelets and nanorods.

 

 

 

SYNTHESIS OF GOLD NANOPARTICLES

 Turkevich method

This method of gold nanoparticle synthesis was devised by someone known as J. Turkevich in the year 1951, and was later refined by G. Frens during the 70s. It is easily the simplest one of all gold nanoparticle synthesis methods.It's primarily used for production of modestly mono disperse spherical gold nanoparticles (of around 10 nm to 20 nm diameter) suspended in water. It's possible to produce larger particles too, but not without compromising on the nanoparticles' shape and mono disparsity.

This method involves reaction of tiny amounts of hot H[AuCl₄] or chloroauric acid with equally little amounts of Na₃C₆H₅O₇ or trisodium citrate solution. Such a reaction leads to creation of gold nanoparticles since the citrate ions function both as a capping and a reducing agent. Larger nanoparticles can be obtained by adding less of trisodium citrate, as less as 0.05%.

Brust method

This method was developed by Brust and Schiffrin during the early 90s.It's actively used for the production of gold nanoparticles in organic liquids,which aren't normally miscible(like toluene) with water.In this method H[AuCl₄] or chloroauric acid solution is reacted with TOAB or trtraoctylammonium bromide solution.The reaction happens in sodium borohydride (NaBH₄) and toluene, which function as areducing agent and anticoagulent respectively.The gold nanoparticles produced by this techniues are 5 nm to 6 nm in diameter.

Green methods

 Green chemistry synthesis routes are environment friendly and non-toxic. A facile green biosynthesis method for the preparation of gold nanoparticles of size 25 + 7 nm was reported by using natural biomaterial egg shell membrane (ESM). In this method ESM was immersed in aqueous solution of H[AuCl₄] without using any reductant. Another green synthetic approach was developed to synthesized gold sononanoparticles of size 5 - 17 nm by using high-power ultrasounds and sodium dehydrate. Gold nanoparticles were successfully synthesized by adopting sun light irradiation method and were modified with folic acid and capped by 6-mercaptopurine. In this method solar energy was used to reduce the gold salt. A new green chemistry method for the preparation of gold nanoparticles has been reported, in which gold nanoparticles were formed in aqueous NaCl solution from the bulk gold substrate by natural chitosan without using any external stabilizer and reductant. Gold nanoparticles of size 15 - 80 nm are also synthesized via another green synthetic route. In this method H[AuCl₄] was reduced by using citrus fruits juice extracts [Citrus limon, Citrus reticulate and Citrus sinensis] . Edible mushroom was also used for the synthesis of gold nanoparticles via sunlight exposure.

PROPERTIES

 

 1. Surface Plasmon Resonance (SPR)

When gold nanoparticles are exposed to a specific wavelength of light, the oscillating electromagnetic field of the light induces a collective coherent oscillation of the free electrons, which causes a charge separation with respect to the ionic lattice, forming a dipole oscillation along the direction of the electric field of the light. The amplitude of the oscillation reaches the maximum at a specific frequency, called surface plasmon resonance (SPR).

When the size of gold nanoparticles change from 10 nm to 50 nm, the maximum extinction of the SPR Band shifts from 517 nm to 532 nm in the visible region, which indicates that the fluorescence intensity and the absorption band of gold nanoparticles are concentration and particle size dependent. In addition, SPR enhances the radiative properties such as absorption and scattering, offering multiple applications for biological and medical applications. For example, gold nanoparticles/titanium dioxide system has been proved to be efficient photo catalyst for some reactions.

 

2. Electrochemical Properties

 

The electrical property of gold particles has been intensively studied in past few years. Electron transport is not confined to the discrete energy levels of several atoms but appears as a continuum energy level. Therefore, surface charging and electron transport processes in gold nanoparticles may be understood with relatively simple classical physical expressions, as for resistance/capacitor electronic circuit diagrams. The electrical property of gold nanoparticles only depends on their size and surrounding medium, which has been used for many applications, such as electrical biosensors and electronic chips.

APPLICATIONS OF GOLD NANOPARTICLES

 

1. Therapy

Gold nanoparticles can strongly absorb light as the result of the SPR. The absorbed light can efficiently be converted to heat by the fast electron–phonon and phonon–phonon processes, which makes gold nanoparticle a useful tool for photothermal therapy of cancers or other diseases. For example, when excited by light at wavelengths from 700 to 800 nm, near-IR absorbing gold nanoparticles can produce heat and eradicate tumors. Based on the enhanced permeability and retention effect and the explosion to the appropriate laser light, gold nanoparticles can be precisely accumulated in tumor cells and targeted treat tumor, which contribute much to the “see and treat” approach.

 

2. Drug and Gene Delivery

Gold nanoparticles conjugated with therapeutic agents improve the pharmacokinetics of the “free” drug and provides controlled or sustained release properties, which makes them an attractive tools for drug delivery and gene delivery. The large surface area-to-volume ratio of gold nanoparticles enables their surface to be coated with hundreds of molecules, including therapeutics, targeting agents, and anti-fouling polymers. Especially, DNA combined assembly gold nanoparticles have been successful used as efficient gene transfection tools.

 

3. Probes

Gold nanoparticles are widely used probes for immunogold staining in transmission electron microscopy (TEM) due to their high electron density. Additionally, the significant SPR-based light scattering capability of gold nanoparticles makes them probes for dark-field microscopy and Raman spectroscopy. Various studies has proved gold nanoparticles to be effective probes for cancer imaging based on their two-photon luminescence imaging, light-scattering imaging, surface-enhanced Raman scattering applications.

4. Sensors

Gold nanoparticles have also been used as colorimetric probes. Typically, gold nanoparticle biosensing is based on the interaction of cross-linker with a receptor molecule on nanoparticles or the interaction between nanoparticles containing receptors when an ligand added in. Especially, gold nanoparticles protected by bovine serum albumin have been introduced as the ratiometric fluorescent probe for in vivo detection. This strategy could also be applied for the detection of proteins, pollutants, and other label-free molecules.

 

 

5. Diagnostics

 

Gold nanoparticles are used as contrast agents in the diagnosis of heart diseases,  cancers, and infectious agents. For example, X-ray computer tomography (CT) is a common diagnostic imaging tool for gold nanoparticles in vivo detection, which is used to visualize tissue density differences that provide image contrast by X-ray attenuation between soft tissues and electron-dense bone. Gold nanoparticles also exhibit good signal intensity and stability when acting as the promising materials for NIR imaging.

Gold nanoparticles are the most commonly used nanoparticles for lateral flow assays. Due to the optical properties of gold nanoparticles, detection with the naked eye can be achieved with excellent sensitivity. The assay can also be adapted to run both in non-competitive and competitive mode.

 

 

 

 

 

 

6. Photocatalyst

 

Gold nanoparticles are used as photocatalysts in a number of chemical reactions. Because of the unique surface plasmon resonance property, the surface of gold nanoparticles can be used for selective oxidation or reduce a reaction in certain cases. Normally, gold nanoparticles are raised as photocatalyst with the combination of titanium dioxide, which can be useful in the chemical industry.

 

 

Gold nanoparticles consider as a promising future for scientists and researchers, especially in the medical field. To see gold nanoparticles being responsible for such significant developments, whether it’s destroying viruses, fighting cancer, identifying deadly conditions or immunising against infections diseases, is hugely encouraging. But what is even more incredible is that we are only now starting to fully realise the potential of the gold nanoparticles and if we take the appropriate approach it will give us more room for discoveries and applications.